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Overview
Automatic Stereo Synthesizer
The 275A reprocesses mono material into synthesized stereo to "open up" mono programs when heard on stereo or surround TV systems. The sound of the 275A is remarkably convincing, producing a pleasing sense of ambience and spaciousness while maintaining perfect mono compatibility. Placed permanently in the program line, the 275A automatically determines if the program is mono or stereo and then smoothly cross-fades from bypass to synthesized stereo output when true stereo program is absent.
Features
Automatic Stereo Synthesizer
- Two methods of automatic mono recognition: "Single-Channel" detects absence of audio on one channel, "Mono/Stereo" recognizes mono in both channels.
- Smooth cross-fading between true and synthesized stereo: no pops, clicks, or discontinuities.
- Full mono compatibility: sum of synthesized outputs is identical to mono input.
- Patented Orban stereo synthesis technique: provides synthesized stereo without addition of unnatural resonances or "flanging" colorations.
- Automatic detection and correction of polarity-reversed (out-of-phase) stereo inputs.
- Noise reduction for mono material: Typically 10dB reduction of hiss and hum; single-ended.
- Optically-isolated external automation control interface.
- Optional remote control unit duplicates main unit functions.
Specifications
Performance
Frequency Response: ±0.5dB, 30-15,000Hz (Bypass Mode); ±1.0dB, 30-15,000Hz (Synthesize Mode), sum of left and right channels.
Total Harmonic Distortion: +18dBu/600ohm: <0.02%, 30-15,000Hz (Bypass Mode); <0.3%, 30-15,000Hz (Synthesize Mode).
Noise at Output: (30-15,000Hz): <-80dBu (Bypass Mode); <-67dBu (Synthesize Mode).
Installation
Input
Impedance: <100 ohm, balanced bridging. Absolute overload occurs at +26dBu into 600ohm.
Connectors
Audio: Barrier strip (#5 Screw).
User Control Interface: DB-25S jack (accepts DB-25P plug, supplied).
275ARC Remote Control: DB-37S jack (accepts DB-37P plug, supplied).
Output
Load Impedance: >600.
Source Impedance: 30, ±5%, balanced, floating.
Maximum Output Level: >20dBu into 600.
Connectors: Barrier strip (#5 screws), XL-type connectors.
Physical
Power Requirements: 115/230VAC ±10%, 50-60Hz, 9VA. 115V version supplied with "U-ground" grounding-type power cord to USA standards; 230V version supplied with CEE-7/7 plug.
Fuse: 1/4 -amp "T"-type (250mA) Slow Blow for 230V operation.
Mounting: Requires 1 unit (1.75"/4.5 cm) of vertical space in an EIA 19"/48.3cm rack. Depth is 9.625"/24.5cm. Optional 275 ARC Remote Control requires the same space, except depth is 2.25"/5.7cm, including supplied connector.
Weight: 12lb (5.4kg), shipping.
Options 275 ARC 19" rack-mount Remote Control provides duplication of all front panel indicators and function except for the SEPARATION control.
Specifications subject to change without notice.
Notes
Stereo Synthesis
The patented Orban stereo synthesis technique creates a perceived stereo result from a mono signal by dividing the audio spectrum into several frequency bands, then directing these bands alternatively to the left and right channels. It does this by passing a mono signal through a chain of phase shifters to generate an artificial L-R signal, which is then added to and subtracted from the mono to obtain the synthesized channels. The net effect is a "complementary comb filter."
The sum of the two synthesized channels always remains equal to the original mono, ensuring complete mono compatibility.
Because the audio spectrum is divided logarithmically, the undesirable harmonic reinforcement and cancellation that can result from the arithmetic band-splitting found in digital delay-based processors is avoided.
Two types of remote-selectable stereo synthesis effects are available: wide for a dramatic sense of stereo space on music and effects, or narrow to center dialogue more accurately. The recessed SEPARATION control adjusts the amount of inter-channel difference (L-R), to establish the relative width of the stereo image.
To maximize loudness while making efficient use of the modestly-priced amplifiers in most consumer TV sets, energy below approximately 200Hz remains mono. The ear cannot detect separation in this region.
Noise Reduction
Older mono material often suffers from hiss and other forms of noise. The 275A can apply single-ended noise reduction to mono audio prior to stereo synthesis processing. The noise reduction combines program-controlled high-frequency filtering with broadband expansion. 10dB of noise reduction is typically achieved-without unnaturally reducing ambience and dialogue intelligibility when program levels are low.
Because the noise reduction system is single-ended, no encoding (or later decoding) of the program material is necessary. This makes the process ideally suited to noisy optical sound tracks and satellite feeds. Operation is exceptionally smooth and subtle, and "pumping" and "breathing" are entirely absent.
Polarity Correction
In stereo material, it is essential that the two channels be in phase with each other. If they are not, the mono sum signal will be seriously degraded as the two channels cancel each other. And that means that the viewer with a mono set (a majority of your audience) will hear disastrously inferior audio, and in some cases, no sound at all!
To ensure the mono compatibility of your stereo broadcasts, the 275A can act as a "watchdog" over your program line polarity, correcting errors when detected. The detection technique is very reliable and highly resistant to "falsing"- even when subjected to substantial high-frequency phase errors (due to misaligned heads or other mechanical problems) or when monitoring sound tracks containing out-of-phase "surround" energy.
An LED on the front (and remote control) panel lights when a polarity reversal is being corrected. The detection/correction circuit can be activated or defeated at any time.